1. Field of the Invention
The present invention relates generally to a stimulating medical device, and more particularly, to the selection of stimulation channels for application of stimulation by the stimulating medical device.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. In some cases, a person may have hearing loss of both types. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss is often addressed with conventional hearing aids which amplify sound so that acoustic information can reach the cochlea.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. Sensorineural hearing loss occurs when there is damage to the inner ear or to the nerve pathways from the inner ear to the brain. Those suffering from sensorineural hearing loss are thus unable to derive suitable benefit from conventional hearing aids. As a result, hearing prostheses that deliver electrical stimulation to nerve cells of the recipient's auditory system have been developed to provide persons having sensorineural hearing loss with the ability to perceive sound. Such stimulating hearing prostheses include, for example, auditory brain stimulators and cochlear prostheses (commonly referred to as cochlear prosthetic devices, cochlear implants, cochlear devices, and the like; simply “cochlear implants” herein.) As used herein, the recipient's auditory system includes all sensory system components used to perceive a sound signal, such as hearing sensation receptors, neural pathways, including the auditory nerve and spiral ganglion, and parts of the brain used to sense sounds.
Most sensorineural hearing loss is due to the absence or destruction of the cochlear hair cells which transduce acoustic signals into nerve impulses. It is for this purpose that cochlear implants have been developed. Cochlear implants use direct electrical stimulation of auditory nerve cells to bypass absent or defective hair cells that normally transduce acoustic vibrations into neural activity. Such devices generally use an electrode array implanted into the scala tympani of the cochlea so that the electrodes may differentially activate auditory neurons that normally encode different frequencies of sound.
Auditory brain stimulators are used to treat a smaller number of recipients with bilateral degeneration of the auditory nerve. For such recipients, the auditory brain stimulator provides stimulation of the cochlear nucleus in the brainstem.
In applying electrical stimulation to a recipient, medical devices, such as cochlear implants and auditory brain stimulators, typically use a coding strategy in determining the timing and intensity of the stimulation pulses to be applied to each implanted electrode contact. These coding strategies aim to divide the input signal into a number of frequency components using a different band-pass filter for each electrode. However, in order to use separate frequencies for each electrode, the filters are generally narrow and therefore rapid modulations of frequency components corresponding to periodicity in the input signal are largely removed. Periodic modulations in the input signal carry information about the sound source such as voice pitch (i.e., male or female) or the fundamental frequency for musical and other sounds. This loss of information may result in poor coding of the fundamental frequency for both speech and music. However, if wider filters are used to capture this periodic modulation, then the frequency spectrum of the signal may be less precisely characterized, which can exacerbate the overlap of current fields produced by adjacent electrodes (thereby mixing up the signals between electrodes). In addition, if these filters are too wide they could confuse periodic modulations corresponding to the fundamental frequency with those that may erroneously occur due to the combination of even-numbered lower harmonics.